This invention relates to a radiation dosimeter. More particularly, this invention relates to a method for manufacturing a radiation dosimeter. Even more particularly, this invention relates to a manufacturing method wherein radiation sensitive patches of radiation dosimeters are individually calibrated for sensitivity to facilitate eventual use in quantitatively measuring radiation doses.
In facilities where radioactive materials are used, for example, in hospitals where cancer patients receive radiation treatments or in blood banks where blood products are irradiated, various methods are used to quantitatively determine the radiation dose. The methods practiced include the use of thermoluminescent dosimeters (TLD's), ionization-type radiation detectors, photographic film, and radiochromic materials. TLD's are inconvenient because they require a complicated and time-consuming read-out process. Ionization-type radiation detectors are awkward and unwieldy and require a complicated setup. Photographic film requires a time-consuming chemical processing procedure before read-out. Radiochromic materials are inconvenient in current practice because the calculation of the dose requires a complex sequence of steps, subject to operator error.
U.S. Pat. No. 5,637,876 describes a radiation dosimeter, exemplarily for use in determining a level of radiation to which a patient is subjected during radiation treatment, which comprises a substrate provided with a layer of radiation sensitive material. The radiation sensitive material has an optical density which varies systematically in accordance with the degree of radiation exposure. In addition, the substrate is provided with optically readable coding which identifies encoded mathematical parameters for enabling an automated calculation of dosage from a detected post-exposure optical density (or change in optical density) of the radiation sensitive material. Where the post-exposure optical density varies as a linear function of the amount of radiation exposure, the mathematical parameters include a slope parameter and a y-intercept parameter.
According to U.S. Pat. No. 5,637,876, the dosimeter may take the form of a card or a flexible substrate which is positionable on the patient or other irradiation subject and which is also positionable in, or slidable through a slot in, a dose reader, described below. Preferably, the coding on the substrate takes the form of a bar code. In that case, the coding and the optical density of the exposed layer of radiation sensitive material may be read by the same dose reader instrument. The bar coding and the reflection (or transmission) intensity of the radiation sensitive layer may be sensed during a sliding of the dosimeter through a slot on the dose reader instrument. Alternatively, movable optical elements may be provided for reading the bar code information and measuring the optical density of the radiation sensitive layer while the dosimeter is held in a slot or recess on the dose reader instrument.
Also described in U.S. Pat. No. 5,637,876 is a dose reader instrument which is used with the dosimeter in measuring a radiation level to which a patient or other object is subjected and which comprises an optical sensor for sensing a range of variable optical densities of a radiation sensitive layer. The sensor includes or is connected to measurement componentry for determining an optical density of the layer of radiation sensitive material on the substrate. The dose reader further comprises a decoder operatively connected to the optical sensor for decoding the mathematical parameters encoded in the optically readable coding on the substrate. A computer is operatively connected to the measurement componentry and the decoder for computing, according to a predetermined mathematical function including the parameters determined from the coding on the substrate by the decoder, a quantitative radiation dose to which the layer of radiation sensitive material was exposed. A display or other communicating component (such as speech synthesis circuitry) is operatively connected to the computer for communicating the computed quantitative radiation dose to an operator.
As discussed above with respect to the structure of the dosimeter, where the radiation level to which a subject is exposed is linearly related to the change in the optical density of the exposed layer of radiation sensitive material, the mathematical parameters encoded on the dosimeter include a slope parameter and a y-intercept parameter. The predetermined mathematical function used in computing the level of radiation exposure is [log[I(0)-D]-log[I(s)-D]-b]/m where D is a premeasured background intensity determined for the instrument during production and assembly, m is the slope parameter, b is the y-intercept parameter, I(0) is a sensed pre-exposure reflection or transmission intensity of the layer of radiation sensitive material, I(s) is a sensed post-exposure reflection or transmission intensity of the layer of radiation sensitive material, and [log[I(0)-D]-log[I(s)-D]] is a measured optical density change in the layer of radiation sensitive material.
Where another mathematical function describes the relationship between post-exposure optical density change of a radiation sensitive dosimeter layer and the degree of irradiation, different mathematical parameters are encoded on the dosimeter, e.g., in a bar code. The principle underlying the invention of U.S. Pat. No. 5,637,876 is that the calibration information pertaining to the relationship between a post-exposure optical density change of a radiation sensitive dosimeter layer and the degree of irradiation is encoded on the dosimeter itself, thereby enabling automatic computation of the radiation dosage from a measured optical density change.
Pursuant to the disclosure of U.S. Pat. No. 5,637,876, a method for determining a level of exposure to radiation utilizing the radiation dosimeter and dose reader instrument described above comprises the step of optically measuring the pre-exposure optical density of the layer of radiation sensitive material. In addition, the coding on the dosimeter substrate is scanned to automatically determine the encoded mathematical parameters. Generally, after measurement of the pre-exposure optical density of the radiation sensitive layer, the dosimeter is placed on a subject to be irradiated. The method further comprises the steps of then exposing the radiation sensitive layer (and the subject) to radiation and subsequently optically measuring a post-exposure optical density of the radiation sensitive layer. Then, from the pre-exposure optical density, the post-exposure optical density, and the mathematical parameters and in accordance with a predetermined mathematical algorithm, a quantitative radiation dose to which the layer of radiation sensitive material was exposed is automatically computed. Preferably, the computed quantitative radiation dose is automatically indicated on a display.
The optical density of the layer of radiation sensitive material may be measured by sensing a reflection (or transmission) intensity of the layer. The optical density is related logarithmically to the sensed reflection (or transmission) intensity. Where the reflection intensity is sensed, a reflection densitometer may be used.
In practice, the calibration information (e.g., the y-intercept and the slope) printed on any particular dosimeter represents, at best, an average sensitivity for an entire lot of radiation sensitive dosimeter material of which the particular dosimeter is a part. Calibration information is currently derived, in a proprietary process, by exposing samples of a production batch of dosimeter material (radiation sensitive layers) to known levels of radiation, calculating their specific responses (density changes per known doses), and computing an average of the specific responses. The average values are printed in code on each dosimeter of the production batch.
Of course, manufacturing tolerances in this calibration process give rise to variances in accuracy of the calibration information printed on the individual dosimeters. The variance of the sensitivity of individual film chips within a lot results in a limitation on the accuracy of radiation dose measurements made with the individual film chips.